1. Field of the Invention
A two-step black and white photographic process in which the developed image is enhanced by controlled bleaching during fixing immediately following development.
2. Description of the Prior Art
The method presently most widely used in black and white photographic processing of latent images in black and white silver halide negative and positive emulsions is a two-step process, in the first step of which a conventional black and white developer suitable for the particular emulsion and conditions is employed, and in the second step of which undeveloped silver halide is removed with a conventional fixer such as a thiosulfate. Such two-step black and white photographic process is used, for example, to form photographic black and white negatives, black and white prints, e.g. paper prints, high contrast continuous tone emulsions such as reprographic emulsions, X-ray films and photomechanical reproductions, both line and screened.
In all of such two-step black and white photographic processes a phenomenon known as fog occurs. This is due to the undesirable reduction of unexposed silver halide to metallic silver, principally, in areas which are supposed to be void of metallic silver. The amount of metallic silver that creates fog is not considerable, but the very presence of the fog reduces contrast and, in films which are to be viewed in front of a light, causes haze. The phenomenon of fog has been considered as an undesirable drawback of black and white photographic developing processes. Because of its supposed inherence, it has been accepted despite the disadvantage of its presence. Indeed, a common measurement employed in evaluating emulsions, and various photographic processes and chemicals has been the lowest level of density created in an unexposed emulsion after developing and fixing; this level has been called "base+fog" simply because fog has been accepted as a necessary evil of the photographic process.
Another problem which existed in the art, and this pertained principally to radiographic films, was that the silver metal formed upon development in unexposed areas of the emulsion caused haze which is a different aspect of fog. Fog, per se, is merely an increase in density which can be read with a densitometer. Haze is a diffusion of light noticeable when a transparency is held in front of a light source. Haze gives rise to a halo around the light source because the unwanted metal silver particles diffuse transmitted light.
It would be highly desirable to eliminate or greatly reduce fog and haze. Such reduction would enhance an image by improving contrast generally through lightening of the areas of a developed image which should be unclouded and by eliminating or reducing haze so that when a radiograph is viewed the combination of the reduction in fog and the reduction in haze would enable a radiologist to far more readily inspect a radiograph and observe details which heretofore were somewhat fuzzy and could have led to doubtful and even erroneous diagnoses.